1. Field of Art
This disclosure relates generally to the field of photovoltaic power systems. More specifically, this disclosure relates to a rooftop shingle-based photovoltaic power system where highly efficient dc-dc converters are included in one or more building-integrated photovoltaic (BIPV) roof shingles.
2. Description of the Related Art
Solar photovoltaic (PV) cells typically produce dc voltages of less than one volt. The amount of electrical power produced by such a cell is equal to its dc voltage multiplied by its dc current, and these quantities depend on multiple factors including the solar irradiance, cell temperature, process variations and cell electrical operating point. It is commonly desired to produce more power than can be generated by a single cell, and hence multiple cells are employed. It is also commonly desired to supply power at voltages substantially higher than the voltage generated by a single cell. Hence, multiple cells are typically connected in series.
For example, consider a conventional rooftop solar power system, such as a 2 kW (grid-tied) rooftop solar PV power system that delivers its power to a 240 V AC utility. Because of the very large number of PV cells required in a conventional rooftop solar power system, the individual PV cells are typically packaged into intermediate-sized panels such as photovoltaic (PV) shingles, which typically include several series-connected PV cells and typically produce several volts DC. PV shingles may be connected in series with the output of the series connected PV shingles connected through a combiner circuit to the input of a central DC-AC inverter, which changes the high DC voltage (e.g., several hundred volts) generated by the series-connected conventional PV shingles into 240 V AC as required by the utility. In addition, the central DC-AC inverter performs certain grid interface functions as required by standards (such as IEEE Standard 1547) and building codes, which may include anti-islanding, protection from ac line transients, galvanic isolation, production of ac line currents meeting harmonic limits, and other functions.
In a conventional system, such as the 2 kW rooftop solar PV power system described above, the DC-AC inverter may include a DC-DC conversion module and an AC interface module. Control circuitry for the DC-AC inverter can implement a maximum power point tracking (MPPT) algorithm, as known in the art. The DC-DC conversion module includes DC-DC conversion circuitry and can serve as a central DC-DC converter for the output of multiple PV shingles. Control circuitry within the DC-AC inverter can control the DC-DC conversion module to adjust the voltage at the input to the DC-AC inverter to maximize the power flowing through the DC-AC inverter. The DC-AC inverter also includes an AC interface module, typically a DC-AC converter, to interface to an AC utility grid.
Because power produced by a PV shingle depends on the voltage and current of the PV shingle and on other factors including solar irradiation and temperature the maximum current that a PV shingle can produce (the “short circuit current”) is proportional to the solar irradiation incident on a PV shingle. When PV shingles are connected in series, each of the PV shingles must conduct the same current (the “string current”). If a first PV shingle in a series string is partially shaded, then the current of all PV panels in the string is affected. In some instances, the series string operates with a reduced current determined by the current of the shaded conventional PV shingle, reducing the power generated by all PV shingles in the string. Alternatively, the string may conduct a larger current, causing a bypass diode of the shaded PV shingle to conduct and prevent power from being harvested from the shaded PV shingle, reducing the total voltage produced by the string. In either case, the PV power generation system produces less than the maximum possible power.
Additionally, the DC-DC conversion module included in the DC-AC inverter typically operates with less than 100% efficiency, and some fraction of the power generated by the collection of PV shingles (referred to as a photovoltaic array) is therefore lost.